1. Field of the Invention
The present invention relates to optical semiconductor device, and more particularly, to optical semiconductor device having a optical semiconductor element and a circuit connected thereto and used under the environment where light is irradiated.
2. Discussion of the Background
Optical semiconductor devices, such as an optical sensor, photo-coupler, photo-interrupter, or a device for an optical link system include an optical semiconductor element such as a photo-detecting element and a light emitting element. In many cases, a circuit connected to the optical semiconductor element is also provided in such an optical semiconductor device.
The circuit supplies a voltage or a current for driving the optical semiconductor device. Moreover, the circuit may also have a role to process the signal which is emitted from the optical semiconductor device or accepted on the optical semiconductor device.
In some cases, the circuit may include a circuit where two or more rectifiers are connected in series.
FIG. 16 is a schematic diagram illustrating such a circuit. This figure shows a circuit in which the n zener diodes 1 are connected in series. The zener diode 1 is an active element where the breakdown phenomenon of a p-n junction is utilized.
The breakdown voltage Vz of the zener diode is usually around 5 volts when it is made of silicon (Si). Then, the series connection circuit of the zener diodes 1 shown in FIG. 16 can be used as a protection circuit for protecting optical semiconductor devices including a photo-detecting device and a light emitting element from destruction by ESD (Electro-Static Discharge).
Moreover, the series connection circuit of zener diodes 1 can be also used as a standard power supply of a series regulator in a power supply for driving an optical semiconductor element, and as a clamp circuit for protecting a gate of a MOS transistor.
Since the breakdown voltage of Vz of each zener diode 1 is about 5 volts, high voltages, such as 30 volts and 40 volts, can be obtained easily. Such a series circuit can also clamp a voltage. Therefore, when the chip size of IC is restricted, it is convenient to use the zener diodes 1. For example, when clamping 30 volts, six zener diodes 1 may be employed and connected in series.
In the case of the circuit illustrated in FIG. 16, n zener diodes 1 are connected in series, and a power supply E1 is connected between the both ends, that is, between the terminal PK and the terminal PA. This power supply is virtual, and for deciding the potentials of the terminals of the series circuit of n zener diodes 1, and may not necessarily be an external power supply.
A zener diode 1 is made on p type silicon substrate grounded through the terminal PS, as mentioned after. Further, a power supply E2 is connected between the terminal PA and the terminal PS. Therefore, the potential of the terminal PK becomes the highest in this circuit.
In this circuit, the relation with power supplies E1 and E2 is always represented by the following formula:E1≧E2Even if a power supply E1 changes, unless power supplies E1 and E2 have the above (n×Vz) potential difference, current does not flow through the zener diodes 1.
If the potential difference beyond it arises between the power supplies E1 and E2, current will flow through the zener diodes 1 and the voltage between the terminal PK and the terminal PA will be clamped at (n×Vz) volts.
FIG. 17 is a schematic diagram illustrating the example of a series clamp circuit in which n zener diodes 1 are used. Here, the ESD (Electro-Static Discharge) clamp circuit 6 is inserted between the IC output terminal Vo and the constant voltage terminal Vdd.
This clamp circuit operates at the voltage more than Vdd. To meet this need, the n zener diodes 1 are connected in series. And if the potential difference between the Vdd terminal and the Vo terminal becomes above (n×Vz), the clamp circuit begins to operate. In the case of this circuit, the Vdd terminal corresponds to the terminal PK in FIG. 16, and the base of the NPN-transistor 20 corresponds to the terminal PA in FIG. 16.
A surge absorption circuit with zener diodes provided in a regeneration control system for inductive load is disclosed in Japanese Patent Laid-Open Disclosure No. 10-136564. The diode for preventing an adverse current is connected in series to the zener diode as a surge absorption element in this surge absorption circuit.
However, it is thought that this diode for preventing the adverse current is for preventing that a current flows from a power supply to FET in the state of ON of FET.
Another circuit for controlling a regeneration voltage is disclosed in the U.S. Pat. No. 4,658,203. In this circuit, a feedback circuit having a zener diode and an opposite direction diode is inserted between a drain and a gate of a switching element which controls a motor as a load. The opposite direction diode has a role to prevent the feedback circuit from turning on at the time of the normal operation of the switching element.